In recent years, certain high performance bicycles have included hydraulic disc brakes. Hydraulic disc brake systems typically include a caliper housing, a first movable brake pad and a second fixed or movable brake pad. The movable brake pad is typically attached to a piston that is movable in response to fluid pressure applied via a hydraulic fluid conduit in the caliper housing. The brake pads are positioned on either side of a rotor, which is attached to the front or rear wheel of a bicycle. Upon the application of fluid pressure to the piston or pistons, the brake pads come into contact with the rotor, thereby applying frictional resistance and causing the bicycle to slow down or stop.
Hydraulic disc brake systems for bicycles are typically actuated by a brake lever attached to a bicycle handlebar. They also typically include a master piston in a master cylinder which is actuated by the brake lever. The master cylinder contains a hydraulic fluid and is in fluid communication with the disc brake caliper via a fluid conduit. The brake pads are typically spaced apart from the rotor by a predetermined gap. As the lever is contracted towards the handlebar, the master piston moves, thereby forcing liquid out of the master cylinder and into a conduit connected to the caliper housing. The movement of fluid into the caliper housing causes the pistons to move, eventually bringing the brake pads into contact with the rotor. Once the brake pads contact the rotor, they provide frictional resistance which can be increased by further operation of the lever. At this point, the caliper housing is fully pressurized, and further operation of the lever increases the system hydraulic pressure and frictional resistance applied to the rotor.
However, users of hydraulic disc brake systems that include only a single master piston often find it difficult to control braking after the brake pads contact the rotor because the hydraulic pressure is directly affected by the master piston and nothing else. Therefore, after the brake pads contact the rotor, if the user grips the brake lever harder, thereby causing the hydraulic pressure to increase as a result of the movement of the master piston, the brakes often lock, causing the tire to skid. This is undesirable.
Other hydraulic brake systems have been developed that include two pistons in the master cylinder. For example, see U.S. Patent Application No. US-2006-0185360-A1 to Takizawa et al., the entirety of which is incorporated by reference. This type of hydraulic brake system includes a brake lever that is pivotally attached to a brake housing, a master cylinder in the brake housing that has two fluid containing regions of different diameter and a master piston assembly that has two pistons. The first piston is generally annular in shape and includes a central opening through which a second piston extends. The first and second pistons have different diameters that each correspond to the diameter of one of the two fluid containing regions of the master cylinder.
However, this type of hydraulic brake system has similar braking problems to the single master piston design described above. In this system, the added hydraulic pressure provided by the second piston after the brake pads contact the rotor often causes the brakes to lock and the tire to skid.
Accordingly, a need exists for a hydraulic disc brake lever that addresses the foregoing problems.